Cellular networks hold out the promise of a continuously updated locational data stream for a very large sample of a city's residents and visitors. As a complement to traditional methods of data collection to support planning and policy making, data generated as a byproduct of network operations can provide us with a dynamic view of urban activity, greatly augmenting our understanding of the urban environment at low cost and with little latency. This exploratory article assesses the feasibility of various analytical approaches through a small sample of points from across the city of Rome. The results highlight directions for future study and demonstrate some shortcomings of the existing data sets and analysis. This article is part of a special issue on Urban Computing.